I want to take my baby sister away from her mother....

I want to take custody of my baby sister (2yrs old), her mother is on drugs and look very horrible and I’m afraid that something bad will happen to her, I’m 18 years old, I’m in the process of getting a car, I’m in college M-THUR (M&W-2-3:30pm) (Tues&Thur-7-9:45pm), all I need to do is find a babysitter while I’m at school, I have the means to take very good care of her, but I live with my grandparents as of right now because I’m in school and my grandma doesn’t want me to keep her because she cries at night, and we don’t know if it’s my father’s baby but honestly I don’t care, I just want her safe, what should I do?